1. Technical Field
This application generally relates to computer systems, and more particularly to computer data storage.
2. Description of Related Art
Computer systems may include different resources used by one or more host processors. Resources and host processors in a computer system may be interconnected by one or more communication connections. These resources may include, for example, data storage devices such as the Symmetrix™ family of data storage systems manufactured by EMC Corporation. These data storage systems may be coupled to one or more host processors and provide storage services to each host processor. An example data storage system may include one or more data storage devices, such as those of the Symmetrix™ family, that are connected together and may be used to provide common data storage for one or more host processors in a computer system.
A host processor may perform a variety of data processing tasks and operations using the data storage system. For example, a host processor may perform basic system I/O operations in connection with data requests, such as data read and write operations and also administrative tasks, such as data backup and mirroring operations.
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units, disk drives, and disk interface units. Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may nor correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
A computer system may utilize techniques in connection with data storage to provide for multiple copies of data that are as current as possible to be used, for example, in the event of a system disaster at a data storage site.
Referring to FIG. 50, shown is an example of a prior art system 2000 utilizing data replication techniques. The host 2002 communicates with the primary data storage 2004a to perform data processing operations, such as data read and write operations. The system 2000 includes a mirror copy of the data of device 2004a in device 2004b. The mirror copy 2004b may be stored locally, or within close proximity to device 2004a, rather than at a remote other geographic location. The device 2004a may be a master storage device having one or more slave storage devices, such as 2004b and 2004c. Device 2004c may be remotely located with respect to 2004a and 2004b. The data storage devices 2004a-2004c may communicate using the Remote Data Facility (RDF) product as provided by EMC Corporation. The devices 2004a-2004c may be one or more Symmetrix™ data storage systems. RDF may be used facilitate copying of data between storage devices. The host 2002 interacts with the device 2004a but data updates may be automatically propagated to devices 2004b and 2004c using RDF. The devices 2004a-2004b and 2004b-2004c may be connected by a data link, such as an ESCON link or Fibre Channel link. The RDF functionality may be facilitated with an RDF adapter (RA) provided within each storage device 2004a-2004c. 
When 2004b is remote with respect to 2004a, devices 2004a and 2004b communicate in a synchronous mode such that the host receives an acknowledgement that a write operation is complete when it has been written to both devices. Device 2004a may send the host an acknowledgement that the data operation is complete after receiving a completion acknowledgement from device 2004b. Devices 2004b and 2004c may communicate using an asynchronous communication mode.
It may be that the copy of data to be used, for example, in connection with performing data recovery operations, is remotely located, such as device 2004c, rather than locally located, such as device 2004b. Communications with such remote devices may performed asynchronously, as described above. If a remote device is used in connection with, for example, data recovery operations, the remote device 2004c may not contain an up-to-date copy of the data of device 2004a. 
Another technique may utilize Business Continuance Volumes (BCVs). BCVs, and data operations used in connection therewith, are described in U.S. Pat. No. 6,101,497, filed on Apr. 25, 1997, which is incorporated by reference herein. Referring again to FIG. 50, the data storage system 2004a may include a BCV device for each standard device to which the host 2002 may perform data operations. A standard device may be characterized as a device configured for use with host applications. The BCV may be attached to a standard device. Subsequently, the BCV may be split consistently from the associated STD device to provide a copy of the data on the BCV device for some other purpose. In other words, after the BCV is attached and achieves a state of synchronism with the standard device R1, the BCV may be split or detached from the standard device. The copy of data on the BCV device may then be available for use by other applications, for example, an ancillary application such as a backup or remote copy application that copies data from the device 2004b. The BCV allows for a consistent copy of data on the standard device to be maintained and then independently allows another application to act on the data copy while letting the host perform continued data operations to the standard device. However, this technique may also have the drawback that the copy of the data on the device 2004b may not be an up-to-date copy of the data in the standard device.
Thus, it may be desirous and advantageous to have an efficient technique which provides automated data replication processing to provide one or more consistent copies of data wherein a copy of data is as current as possible. It may also be desirable and advantageous to provide a copy of data that is restartable from a remote data storage site different from a local data storage site, for example, in the event of a disaster at the local data storage site. It may be desirable that such techniques be automated and may be used in connection with one or more groups of devices.